1. Field of the Invention
The present invention relates generally to enlarging the diameter of a subterranean borehole and, more specifically, to enlarging the borehole below a portion thereof which remains at a lesser diameter. The method and apparatus of the present invention includes the enhanced capability to stabilize a reaming tool.
2. State of the Art
It is known to employ both eccentric and bi-center bits to enlarge a borehole below a tight or undersized portion thereof.
An eccentric bit includes an eccentrically, laterally extended or enlarged cutting portion which, when the bit is rotated about its axis, produces a borehole larger than the overall diameter of the eccentric bit. An example of an eccentric bit is disclosed in U.S. Pat. No. 4,635,738.
A bi-center bit assembly employs two longitudinally superimposed bit sections with laterally offset longitudinal axes. The first axis is the center of the pass-through diameter, that is, the diameter of the smallest borehole the bit will pass through. This axis may be referred to as the pass-through axis. The second axis is the axis of the hole cut as the bit is rotated. This axis may be referred to as the drilling axis. There is usually a first, lower and smaller diameter pilot section employed to commence the drilling and rotation of the bit centered about the drilling axis as the second, upper and larger diameter main bit section engages the formation to enlarge the borehole, the rotational axis of the bit assembly rapidly transitioning from the pass-through axis to the drilling axis when the full-diameter, enlarged borehole is drilled.
Rather than employing a one-piece drilling structure such as an eccentric bit or a bi-center bit to enlarge a borehole below a constricted or reduced-diameter segment, it is also known to employ an extended bottomhole assembly (extended bi-center assembly) with a pilot bit at the distal or leading end thereof and a reamer assembly some distance above. This arrangement permits the use of any bit type, be it a rock (tri-cone) bit or a drag bit, as the pilot bit. Further, the extended nature of the assembly permits greater flexibility when passing through tight spots in the borehole as well as an opportunity to effectively stabilize the pilot bit so that the pilot hole and the following reamer will take the path intended for the borehole. This aspect of an extended bottomhole assembly is particularly significant in directional drilling.
While all of the foregoing alternative approaches can be employed to enlarge a borehole below a reduced-diameter segment, the pilot bit with reamer assembly has proven to be highly effective. The assignee of the present invention has, to this end, designed as reaming structures so-called xe2x80x9creamer wingsxe2x80x9d in the very recent past, which reamer wings generally comprise a tubular body having a fishing neck with a threaded connection at the top thereof and a tong die surface at the bottom thereof, also with a threaded connection. As an aside, short-bodied tools frequently will not include fishing necks, including the short-bodied reamer wings designed by the assignee of the present invention. The upper midportion of the reamer wing includes one or more longitudinally extending blades projecting generally radially outwardly from the tubular body, the outer edges of the blades carrying superabrasive (also termed xe2x80x9csuperhardxe2x80x9d) cutting elements; commonly, such superabrasive cutting elements, or cutters, are frequently comprised of PDC (Polycrystalline Diamond Compact) cutters. The lower midportion of the reamer wing may include a stabilizing pad having an arcuate exterior surface sized the same as or slightly smaller than the radius of the pilot hole on the exterior of the tubular body and longitudinally below the blades. The stabilizer pad is characteristically placed on the opposite side of the body with respect to the reamer wing blades so that the reamer wing will ride on the stabilizer pad due to the resultant force vector generated by the cutting of the blade or blades as the enlarged borehole is cut.
Notwithstanding the success of the aforementioned reamer wing design, it was recognized that such devices constructed as described above might not effectively and efficiently address the problem or task of achieving a rapid transition from pass-through to full-hole or xe2x80x9cdrillxe2x80x9d diameter which closely tracks the path of the pilot bit and which does not unduly load the blades or bottomhole assembly during the transition. Since a reamer wing may have to re-establish a full-diameter borehole multiple times during its drilling life in a single borehole, due to washouts and doglegs of the pilot hole, a rapid transitioning ability when reaming is restarted as well as a robust design which can accommodate multiple transitions without significant damage was recognized as a desirable characteristic and design modification. U.S. Pat. No. 5,497,842, assigned to the assignee of the present invention and the disclosure of which is incorporated herein by reference, discloses the use of so-called xe2x80x9csecondaryxe2x80x9d blades on the reamer wing to speed the transition from pass-through to drill diameter with reduced vibration and borehole eccentricity.
While the improvement of the ""842 patent has proven significant, it was recognized that further improvements in the overall stability of the bottomhole assembly, including transitioning from pass-through diameter to drill diameter, would be highly desirable. One problem the prior art reamer assembly designs have experienced is undue vibration and even so-called bit xe2x80x9cwhirl,xe2x80x9d despite the focused or directed force vector acting on the reaming assembly and the presence of the stabilization pad. These undesirable phenomena appear to be related to the configuration of the stabilization pad (illustrated in FIG. 5 of the ""842 patent), which engages the borehole wall axially and circumferentially under the radially directed resultant force vector of the reamer wing as the assembly drills ahead in the pilot hole, due to the pad""s abrupt radial projection from the reamer wing body. Furthermore, it was observed that the entire bottomhole reaming assembly as employed in the prior art for straight-hole drilling with a rotary table or top Be drive often experiences pipe xe2x80x9cwhipxe2x80x9d due to lack of sufficient lateral or radial stabilization above the reamer wing. In addition, such reaming assemblies driven by downhole steerable motors for so-called directional or navigational drilling sometimes experience problems with stability under the lateral forces generated by the reamer wing so as to make it difficult to maintain the planned borehole trajectory.
U.S. Pat. No. 5,765,653, assigned to the assignee of the present invention and the disclosure of which is incorporated herein by reference, addresses the aforementioned problems by providing an axially as well as circumferentially tapered pilot stabilizer pad (xe2x80x9cPSPxe2x80x9d) (see FIGS. 4, 6, 7 and 7A of the ""653 patent) to the reaming apparatus.
U.S. Pat. No. 5,957,223, assigned to the assignee of the present invention and the disclosure of which is incorporate herein by reference, also addresses stability of reaming tools. Specifically, the resultant lateral force vector generated via the pilot bit cutting elements is substantially radially aligned with the much larger lateral force vector generated by the reamer bit section. These two aligned force vectors thus tend to press the bit in the same lateral direction (which moves relative to the borehole sidewall as the bit rotates) along its entire longitudinal extent so that a single circumferential area of the pilot bit section gage rides against the sidewall of the pilot borehole, resulting in a reduced tendency for the bit to cock or tilt with respect to the axis of the borehole.
Furthermore, U.S. Pat. No. 6,116,356 assigned to the assignee of the present invention and the disclosure of which is incorporated by reference herein, provides a pilot stabilization pad (PSP) with an axially and circumferentially tapered, arcuate lower entry surface of increasing diameter as it extends upwardly and away from the direction of bit rotation, in combination with a contiguous, circumferentially tapered, transition surface gradually extending to a greater diameter with respect to the centerline of the reaming tool body opposite the direction of tool rotation. In addition, PSP placement may occur at one or more locations both longitudinally and circumferentially above the reaming apparatus. The PSP stabilizes a reaming assembly by contacting the pilot borehole, thus counteracting forces encountered by the reamer wing during reaming. Circumferential placement of the PSP may be determined by the resultant lateral force vector generated by the blades of the reamer. Thus, optimally, the PSP maintains intimate, stable, and substantially continuous contact with the wall of the pilot borehole not only during entry of the PSP into the pilot hole but also thereafter during the hole opening process.
Unfortunately, one remaining problem with the use of state of the art reaming apparatus is that often the pilot hole is slightly oversized for any number of reasons. An xe2x80x9coversized pilot boreholexe2x80x9d as herein used denotes that the pilot borehole diameter is somewhat larger than the diameter of the pilot bit or other tool used to create the pilot borehole. Usually, the diameter of the pilot borehole is intended to be substantially identical to the diameter of the tool used to create the pilot borehole. Thus, the stability advantages of the PSP are somewhat compromised when the pilot borehole is oversized, since the additional size of the pilot borehole provides less lateral constraint during rotation than a smaller pilot borehole would provide. In addition, since the PSP is typically rotationally located on a substantially opposing side of the reaming tool in relation to the blades of the reamer wing, the excessive diameter of the pilot borehole allows the reamer wing blades to be displaced inwardly from the desired diameter of the reamed borehole. Therefore, the reamer wing drills an undersized reamed borehole in response to the pilot borehole being oversized.
The present invention includes apparatus and methods addressing recognized problems associated with reaming when an oversized pilot borehole occurs. More specifically, the present invention configures a reaming tool so that the PSP may provide stability in a pilot borehole which is oversized relative to the physical pilot bit diameter used to drill the pilot borehole so as to ream the desired borehole size or diameter. The PSP as well as the pilot bit may be modified in size or configuration or laterally offset, angularly offset, or both, to enable the PSP to provide enhanced stability for reaming the pilot borehole size or diameter to its intended magnitude. In one embodiment, an outer bearing surface of the PSP may be sized so that the rotational diameter traversed by the PSP may be at least the same as, or even greater than, the physical diameter of the pilot bit. In any case, in contrast to conventional reaming tools, the reaming tool of the present invention may be used to effect a reamed borehole diameter which is larger than a physical diameter of the reaming tool by taking advantage of, rather than seeking to avoid, a pilot bit drilling a pilot borehole which is oversized with respect to the pilot bit""s physical diameter.
The present invention includes, without limitation, configuring a reaming tool to provide improved stabilization in a pilot borehole which is oversized with respect to the pilot bit physical diameter. Specifically, one or more of the pilot bit, bit sub, PSP, or other reaming tool element or component may be laterally offset from a conventional position in order to provide enhanced stability to the reaming tool. For instance, an undersized pilot bit may be laterally offset or configured to provide a borehole that may closely match the PSP diameter during reaming while drilling. Alternatively or additionally, the PSP may be laterally offset, sized or configured to closely correspond to the diameter of an oversized pilot borehole drilled by the pilot bit. As an example, the PSP may be offset with respect to the pilot borehole axis, thus enabling engagement of the offset PSP with the wall of the oversized borehole drilled by the pilot bit. As another example, the PSP may be sized with an outer bearing surface extending radially so that, when the reaming tool is rotated, the PSP sweeps a diameter the same as or greater than that of a coaxially rotating pilot bit.
Further, the pilot hole assembly attached to the reamer wing or other reamer portion of the reaming tool may be laterally or angularly offset from the reaming axis of the reamer wing. Doing so causes the pilot bit to drill an oversize pilot borehole, the size of the pilot borehole being determined by the amount of offset as well as the pilot bit diameter. The PSP may also have an offset bearing surface placed and circumferentially located to accommodate the offset of the pilot hole assembly for substantially continuously contacting the pilot borehole wall. In addition, a gage area of the pilot drill bit may be configured to substantially continuously contact the wall of the pilot borehole as well.